Carrier wave signaling system



July 25, 1939. c. E. STRONG ET A1. 2,167,535

CARRIER WAVE SIGNALING SYSTEM l Filed Jan. ll, 1936 2 Sheets-Sheet 1 Fig. 1. I

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July 25, 1939. 5 .c E. STRONG ET A1. 2,167,535

CARRIER WAVE I S IGNALING SYSTEM Filed Jan. 11, 1936 2 Sheets-Sheet 2 MRAP/vf Patented July 25, 1939 UNITED STATES PATENT OFFICE CARRIER WAVE SIGNALING SYSTEM Application January l1, 1936, Serial No. 58,765 In Great Britain February 1, 1935 4 Claims. (Cl. 179-171) This invention relates to carrier wave communication systems and particularly to radio telephone transmitters. It has for its object to reduce the primary power consumption of transmitters with- 5`r out degrading appreciably the standard of service afforded.

More specifically the invention aims at applying the variable carrier method of operation to radio transmitters in an improved manner.

; In the accompanying drawings Fig. l shows schematically a known arrangement for operating with variable carrier Whilst Figs. 2 and 3 illustrate two embodiments of our invention; and Fig. 4 illustrates another embodiment somewhat 1,57, similar t0 that of Fig. 3.

In a radio transmitter operating With variable carrier or, as it is sometimes called, iioating carrier, a high frequency amplifier is modulated simultaneously With speech frequency voltages and zo,v with syllable frequency voltage, that is a unidirectional voltage varying at syllable frequency in accordance with the mean amplitude level of the speechfrequency currents. One method of accomplishing this, which is in use at the present time, is

shown in Fig. l in which i is a high frequency amplifier which is plate modulated by audio frequency and syllable frequency voltages; E is a bank of one or more modulator valves acting as a class Aampliler, the grids of which are excited by audio frey quency and syllable frequencyvoltages, 3is a rectilier producing syllable frequency voltages by the rectification of the speech frequency currents, and 4 is a source of fixed high tension anode supply voltage. The voice frequency voltages on the 3,5. grids of the modulator valves are amplified in -those valves and cause audio frequency currents to pass through the circuit comprising the plate to filament resistance of the modulated amplifier I and the lay-passing condenser C i, thus setting up 4,0', audio frequency voltages across the plate to filament resistance of the modulated amplifier and so modulating at speech frequency the high frequency output of that amplifier. Similarly, the syllable frequency excitation of the grids of the A,y modulator valves causes amplified syllable frequency currents to flow in the circuit comprising the plate to filament resistance of the modulated amplier and the impedance presented by the high tension supply which is low for syllable fre- -7 quencies, setting up syllable frequency voltages d across the plate to filament resistance of the modulated amplifier and so modulating the high frequency output at syllable frequency. The final result is that when audio frequency input is 5,5',d absent, the high frequency output from the modulated amplifier is a steady carrier the amplitude of which is determined by the value of high tension supply voltage. If a single tone audio input is new applied, the output from the syllable frequency rectier is a D. C. voltage which when 5- tuating carrier is modulated by the speech frel5` quencies.

The system described above suffers from the disadvantage that the speech and syllable frequency modulator being a class A amplier with a permanent polarising plate current is inherently 20y inefficient in respect of the ratio of the speech frequency and syllable frequency power output compared to the plate input supplied from the high tension source.

It is one object of the present invention to pro- 25 vide a means of applying floating carrier which is consistent with the use of high efficiency class B push-pull amplifiers as the audio frequency modulators.

It is a second object to provide means for per- 3() forming syllable frequency modulation with high eiiiciency at high power with particular reference to syllable frequency plate modulation on the nal stage of high frequency amplification of a transmitter. 36y

In accordance With one feature of the invention in a floating carrier transmitter in which a modulated thermionic valve device is plate modulated by both speech and syllable freqency voltages,

speech frequency voltages are amplified by a 4,0,

thermionic amplifier biased back to class B operation, before being impressed upon the modulated thermionic valve device.

According to a further feature of the invention in a iioating carrier transmitter in which a modu- 4 5.

lated thermionic valve device is plate modulated by syllable frequency voltages, the plate voltage supply source for the modulated valve device is adapted to be controlled by syllable frequency control voltages in such a manner that the voltage 51.0..

supply varies in accordance with the mean amplitude level of speech frequency currents.

Referring now to the arrangement illustrated in Fig. 2, in this circuit the valves V1 and V2 are acting as modulated high frequency amplifiers in H,

At the same time the carrier is l0- a push-pull circuit. The valves V3 and V4 are biased back to act as class B audio frequency amplifier Valves working into transformer T2, the output of which is applied across the valves V1 and V2. The operation of the circuit is briefly as follows:

The average current of a class B amplifier is proportional to the average amplitude of the current output. The pla-te current taken by the two valves V3 and V4, which in this case is taken from the choke coil L1, is the sum of the plate current taken by V3 and V4 and its amplitude is related to the amplitude of the programme to the transmittel'. Depending on the grid bias applied to these valves the total plate current will vary from a small value when no programme is transmitted to the full load value on full programme.

It is well known that when working in the condition for correct operation as a modulated amplifier, the effective impedance of the plate circuits of the modulated amplifier valves considered from the audio frequency viewpoint is that of a pure resistance. The voltage developed across the modulated amplifier is, therefore, directly proportional to the current taken by the modulator valves as in the arrangement shown the total plate current fiows through the modulated amplifier and the modulator valves in series. It follows therefore that, if the circuit is so adjusted that the correct voltage for obtaining the full power ouput from the modulated amplifier valves, is developed across them when the modulator valves are taking their full load current, then when the modulator plate current falls to a low value corresponding to a low programme load, the modulated amplifier plate current and voltage will also fall to a low value and hence the total power consumption and power output are decreased.

It is possible by suitably varying the ratio of no modulation current to full modulation current in the modulator valves to vary over a wide range the ratio of full-modulation power to no modulation power ratio.

Experience shows that a ratio of the order of one to four between no modulation power and full modulation power is satisfactory for broadcasting services, although other power ratios can also be obtained.

As the efficiency and the impedance of the modulated amplifier remain constant, a change in the total plate current of one to two is required to obtain the above effect. Consequently, with a fixed total anode supply voltage and a condition on full load in which the total anode voltage is divided approximately equally between the modulator and the modulated amplifier, it is necessary that the impedance of the modulators should be three times greater on no modulation than on full modulation. With the modulator adjusted to give the correct eiciency on full modulation, this ratio may not be obtained with a fixed grid bias, as this ratio will depend partly upon the characteristics of the valves used. It is desirable therefore, that the grid potential of the modulator should be varied at syllable frequency so that the grid bias is correct for the average programme level at any instant. Referring to Fig. 1 this is accomplished by means of the rectifier 3 with its associated output resistance and condenser. A similar means may be used in connection with the arrangement of Fig. 2.

It is also possible to increase the power ratio between no modulation and full modulation by suitably changing the modulator grid bias at syllable frequency.

The choke coil L1, in conjunction with the capacity C2, prevent modulation of the modulated amplifier directly by audio frequency variations in the total plate current of the modulator valves. C1 is a by-pass condenser and C3 is a blocking condenser inserted to prevent the plate to filament impedance of the modulated amplifier from being short-circuited for direct current by the secondary of the transformer.

The grid bias of the modulated amplifier valve may be obtained from a source of negative grid bias, such as generator or rectifier, or may be obtained by means of a grid leak connected back to its own filament.

A mercury vapour rectifier equipment fur nished with grid control may be arranged so that the output voltage of the rectifier supplies the plate voltage of the modulated amplifier.

This method is illustrated in Fig. 3 in which I is a high frequency amplifier capable of being modulated by voltages impressed on the anodes; 2 is a high efficiency audio frequency modulator in the form of a Class B push-pull amplifier; 3 is a grid-controlled mercury vapour high tension supply rectifier unit connected to a constant A. C. source and supplying anode voltage to the modulated amplifier; 4' is a grid control unit which, according to methods already known, enables the high tension voltage from the rectifier to be varied by phase displacement of A. C. voltage applied to the grid, these phase displacements being regulated by unidirectional voltages fluctuating at syllable frequency voltages; 5 is a rectifier delivering unidirectional syllable frequency voltages resulting from rectification of audio frequency voltages impressed on its input terminals, and 6 is an anode voltage supply equipment for the modulator.

The manner of operation is briefiy as follows. When no audio input voltage is being supplied, the plate voltage on the modulated amplifier, as furnished by the mercury vapour rectifier, is a certain fixed value and a certain fixed carrier power output is delivered by the modulated amplifier.

If a single frequency audio input is now applied at the speech input terminals, a D. C. voltage due to rectification of this tone will appear at the output of the syllable frequency rectifier 5. This D. C. voltage acting on the circuits in the control unit 4' results in a change of the plate voltage furnished by the mercury rectifier 3 to a new and higher value. This causes the carrier delivered by the modulated amplifier to change to a higher value. At the same time the higher carrier is modulated by the audio tone through the action of the modulator 2.

If speech or music is applied at the input terminals, the output from the rectifier 5 is a voltage varying at syllable frequency which, acting through the control unit 4', causes the plate voltage furnished by the rectifier 3 to vary at syllable frequency resulting in fluctuation o1 the carrier delivered by the modulated amplifier at syllable frequency. At the same time the fluctuating carrier is modulated by the speech or music.

The amount by which the carrier is caused to rise with modulation may be chosen arbitrarily. In broadcasting transmitters it is found satisfactory to arrange that the carrier amplitude corresponding to audio input resulting in 100% modulation should be about twice the amplitude existing when no audio input is applied. The

carrier amplitude varies linearly with the plate voltage on the modulated amplifier and, therefore, for a two to one ratio of amplitude the voltage of the mercury rectifier is doubled for the audio input voltage giving full modulation.

The value of the floating carrier method lies in the saving of power effected by reducing the carrier power when modulation is light. In the present case a reduction of the plate voltage on to the modulated amplifier in a two to one ratio results in a reduction of the pow'er drawn from the mains by the mercury vapour rectifier in a four to one ratio approximately. This reduction is accompanied by a change in power factor from about unity to about .5. When the power factor is low the current can be made to lead or lag as desired. In some cases, operation with leading current may be advantageous.

In the arrangement illustrated in Fig. 3 a separate source of anode voltage is shown for the modulator. This is not essential and with suitable precautions the modulator might be fed from the same anode supply as the modulated amplier.

'Ihe invention is not restricted to the precise arrangements described above and numerous other modifications may be made coming within the scope of the appended claims. For example the average plate current of a class B modulator amplifier which fiuctuates at syllable frequency following the mean level of audio frequency voltage may be used as the supply of syllable frequency voltage for operating on the grid of a mercury vapour rectifier supplying the modulated amplifier. This enables a separate rectifier producing syllable frequency voltages to be dispensed with.

Such a system is represented in Fig, 4 which essentially resembles the system of Fig. 3 except that the rectifier 5 for deriving syllable frequency voltages to control phase shift device 4 has been omitted and the phase shift device 4 is shown as being controlled directly by the average plate current of modulator 2.

What is claimed is:

1. In a floating carrier signalling system in which a carrier frequency Wave is modulated by a source of signal frequency, a modulated thermionic tube device, a transmission channel for applying signals from said source to said device, a controllable voltage supply source, connections from said voltage supply source to the plate circuit of sai-d modulated thermionic tube device, amplifier means included in said transmission channel for amplifying the signals from said source of signal frequency before application of said signals to said device, said amplifier means being biased so as to also produce a uni-directional voltage which varies in accordance with the mean energy level of said source of signal frequency, connections for applying Isaid amplied signals to the plate circuit of said device, and means for controlling said voltage supply source in accordance with said uni-directional voltage.

2. In a floating carrier signally system in which a carrier frequency wave is modulated by a source of signal frequency, a modulated thermionic tube device, a source of signal frequency, means for impressing signals from said source of signal frequency upon said modulated tube device, a mercury vapour rectier for supplying plate voltage to said modulated tube device, a grid control device for said mercury vapour rectifier, electron discharge tube means included in said means for impressing signals for amplifying said signals before impressing them on said modulated tube devicey said electron discharge tube means being biased for class B operation whereby there is produced in the output of said electron discharge tube means an average plate current which varies in accordance with the mean level of said signal frequency voltage, and means for applying said variation in said average plate current to said grid control device.

3, In a oating carrier signally system in which a carrier frequency wave is modulated by a source of signal frequency, a modulated thermionic tube device, a source of signal frequency, a thermionic tube amplifier biased to class B operation for amplifying said source of signal frequency, means for impressing said amplied source of signal frequency upon said modulated tube device, an alternating current power source, a rectifier having a grid and connected to rectify current from said power source for supplying plate circuit of said modulated tube device, and means for using the fluctuations in the average plate current of said class B amplifier for controlling said grid to vary the voltage supplied to said device from said rectifier in accordance with the mean energy level of said source of signal frequency.

4. In a floating carrier transmitting system a source of signals, a push-pull electron discharge amplifier biased for class B operation connected to amplify said signals and having a plate current circuit and an audio frequency output circuit partially coinciding with said plate current circuit but distinct therefrom, a modulated thermionic device, an alternating current power source, a rectifier having a grid and connected to rectify current from said power source for supplying plate voltage to said device, coupling means connecting the output of said amplifier to said device, and separate connections between the plate current circuit of said amplifier and said grid for controlling the voltage supplied to said device from said rectifier in accordance with the average current in the plate circuit of said amplifier.

CHARLES ERIC STRONG. FRANCIS CHARLES MCLEAN. 

